Rescue operations for an unknown number of illegal miners trapped inside an abandoned gold mine shaft near Johannesburg in South Africa have been momentarily halted.

The interruption comes after searchers raised concerns on high levels of carbon monoxide and an underground fire, a day after four miners were rescued. The number of missing miners is still unknown.

“At the time we speak, the rescue operation has been interrupted because of the situation underground including smoke and fire that are in the area so we cannot access the areas where the bodies are ,” explained Joseph Mosebenzi Zwane, South Africa’s Minister of Mineral Resources.

Some local media reports in South Africa say that “several” of the miners had fainted in the tunnels after inhaling carbon monoxide. It’s believed some of the missing may have got out on their own and disappeared for fear of being arrested.

Penalties for illegal mining can include fines and jail terms, and a major South African labor group, has called on authorities to consider legalizing and regulating what it called “small-scale mining” as a way to reduce the danger and rid the business of criminal gangs.

The group, known by its acronym COSATU, said the problem would persist as long as many South Africans are “still sitting at the bottom end of the economic pyramid”, according to media reports.